Sedimentation

Sedimentation

Sediments in the aquatic ecosystem are analogous to soil in the terrestrial ecosystem as they are the source of substrate nutrients, and micro- and macroﬂora and -fauna that are the basis of support to living aquatic resources.

Sediments are the key catalysts of environmental food cycles and the dynamics of water quality. Aquatic sediments are derived from and composed of natural physical, chemical, and biological components generally related to their watersheds.

Sediments range in particle distribution from micron-sized clay particles through silt, sand, gravel, rock, and boulders. Sediments originate from bed load transport, beach and bank erosion, and land runoff. They are naturally sorted by size through prevalent hydrodynamic conditions.

In general, fast-moving water will contain coarse-grained sediments and quiescent water will contain ﬁne-grained sediments. Mineralogical characteristics of sediments vary widely and reflect watershed characteristics. Organic material in sediments is derived from the decomposed tissues of plants and animals, from aquatic and terrestrial sources, and from various point and nonpoint wastewater discharges.

The content of organic matter increases in concentration as the size of sediment mineral particles decreases. Dissolved chemicals in the overlying and sediment pore waters are a product of inorganic and organic sedimentary materials, as well as runoff and ground water that range from fresh to marine in salinity.

This sediment/water environment varies signiﬁcantly over space and time and its characteristics are driven by complex biogeochemical interaction between the inorganic, living, and nonliving organic components. The sediment biotic community includes micro-, meso-, and macrofauna and -ﬂora that are interdependent of each other and their host sediment’s biogeochemical characteristics.

Sedimentation is the direct result of the loss (erosion) of sediments from other aquatic areas or land-based areas. Sedimentation can be detrimental or beneﬁcial to aquatic environments. Moreover, sediment impoverishment (erosion or lack of replenishment) in an area can be as bad as too much sedimentation. Sedimentation in one area is linked to erosion or impoverishment in another area and is a natural process of all water bodies (i.e., lakes, rivers, estuaries, coastal zones, and even the deep ocean).

As an example, detrimental effects can be related to the burial of bottom-dwelling organisms and beneﬁcial effects can be related to the building of new substrates for the development of marshes. These natural physical processes will continue whether or not they are inﬂuenced by the activities of humankind.

Morphological changes (physical changes over a large area) to large aquatic systems can also result in major changes in natural sediment erosion and sedimentation patterns. As an example, the change in the size and shape of a water body will result in new water ﬂow patterns leading to erosion or sediment removal from sensitive areas.

Abatement or control of sedimentation can be successful if implemented on a broad land area or watershed scale and is directly related to improvement in land-use practices. Agriculture and forestry (logging) improvements where soil loss is minimized are not only technically feasible: They can be carried out at a moderate cost and with net beneﬁts.

The U.S. Department of Agriculture has a wide range of training and implementation programs for these types of activities. The United Nations Environmental Programme also has global programs, their Regional Seas activities, to guide countries in the management of land-based activities negatively impacting the coastal zone.

Wetlands that separate upland areas from aquatic areas serve as natural ﬁlters for the runoff from the adjacent land. Wetlands thus serve to trap soil particles and associated agricultural contaminants. The construction of natural buffer zones and wetlands replenishment adjacent to logging areas are effective techniques.

Watershed construction activities such as port expansion, water diversions, channel deepening, and new channel construction must undergo a complete environmental assessment, coupled with predictive sediment resuspension and transport modeling, so alternative courses of action and activities to minimize the negative impacts of sedimentation may be chosen.

Sediment impoverishment is equally important in coastal areas, such as coastal Louisiana where twenty-ﬁve to thirty square miles of wetlands are being lost each year. This loss primarily results from the Mississippi River levee system halting the annual natural replenishment of sediments that rebuilds the marsh system.

Engineered water diversion can replace sediment in the natural system to decrease losses due to dams, levees, jetties, and other structures built to control the ﬂow of water and thus sediments. Proper placement of sediments from navigation dredging can also be a useful abatement technique.

Sediments are absolutely necessary for aquatic plant and animal life. Managed properly, sediments are a resource; improper sediment management results in the destruction of aquatic habitat that would have otherwise depended on their presence.

The United Nations Group of Experts on the Scientiﬁc Aspects of Marine Environmental Protection recently recognized that on a global basis, changes in sediment ﬂows are one of the ﬁve most serious problems affecting the quality and uses of the marine and coastal environment.